The present invention relates to a toroidal continuously variable transmission for use as, for example, automotive transmission.
The toroidal continuously variable transmission has an arrangement wherein input and output discs each having a concave raceway surface are so arranged as to have their raceway surfaces in face-to-face relation while rollers are disposed between these raceway surfaces of the discs as pressed thereagainst via oil films under high pressure.
When the input discs are rotated in the toroidal continuously variable transmission thus arranged, power is transmitted from the input discs to the output discs by means of shear force on the oil films present between peripheral surfaces of the rollers and surfaces of the discs. At this time, the surfaces of the rollers and discs are subjected to stress perpendicular thereto and a great shear stress corresponding to the shear force on the oil films. It may be said, therefore, that the surfaces of the rollers and discs of the toroidal continuously variable transmission are under particular severe contact conditions of high temperature and shear stress as compared with a rolling surface of a typical roller bearing which is essentially subjected to a perpendicular stress alone. Under such contact conditions specific to the rollers and discs of the toroidal continuously variable transmission, even a relatively small contact stress tends to cause plastic deformation of surface areas. In order to assure a required service life of the toroidal continuously variable transmission, therefore, the occurrence of the plastic deformation of the surface areas must be suppressed adequately. Furthermore, some contact conditions may entail the occurrence of a large number of cracks in the peripheral surfaces of the rollers. This also dictates the need for the adequate prevention of such cracks for the purpose of ensuring the required service life.
For these reasons, a high-carbon bearing steel such as SUJ2 or the like is used as material for the rollers and discs. Unfortunately, even the rollers and discs formed of the high-carbon bearing steel suffer severe fatigue due to high shear stress, thus resulting in a decreased life.
Regarding the above rollers and the discs, the rollers are constantly in contact with the discs exclusively at axially central portions of their peripheral surfaces, whereas the discs are in contact with the rollers at portions varied depending upon the speed change ratios of the transmission (that is, the discs are varied in diameter of contact circle thereof). Accordingly, the rollers are subject to more severe contact conditions than the discs, thus sustaining heavier fatigue. This leads to a tendency toward the shorter life of the rollers.
In view of the foregoing problem of the prior art, the present invention is directed to the extension of the service lives of the rollers and discs for use in the toroidal continuously variable transmission.
A toroidal continuously variable transmission according to the invention comprises:
an input disc having a concave raceway surface in a side surface thereof;
an output disc having a concave raceway surface opposing the raceway surface of the input disc; and
a plurality of rollers disposed in a toroidal space defined by the raceway surfaces of the input disc and the output disc and rotated in contact with the raceway surfaces via oil films for transmitting torque between the discs by means of shear force on the oil films,
wherein the discs and rollers are formed of a heat-resisting bearing steel and wherein at least a peripheral surface of the roller is subjected to a surface hardening treatment by shot peening thereby having a compressive residual stress of 0.6 to 1.2 GPa and a hardness of HRC 63 to 66 at the outermost surface thereof.
In the toroidal continuously variable transmission thus constructed, the rollers and the discs have superior high-temperature strength and hardness because of the use of the heat-resisting bearing steel. Furthermore, the peripheral surfaces of the rollers, which are exposed to the most severe contact conditions and hence are most susceptible to fatigue, are subjected to a surface hardening treatment by shot peening. Therefore, an adequate strength can be achieved against high shear stress specific to contact surfaces between the rollers and the discs of the toroidal continuously variable transmission. If the compressive residual stress is less than 0.6 GPa, an adequate durability for the rollers and discs of the toroidal continuously variable transmission cannot be achieved. Increasing the compressive residual stress to above 1.2 GPa entails a significant increase of shot peening cost. Likewise, the rollers and discs are low in durability if the hardness is less than HRC 63. Increasing the hardness to above HRC 66 entails the increase of the shot peening cost. Therefore, adjustment is made to limit the compressive residual stress in the range of 0.6 to 1.2 GPa and the hardness in the range of HRC 63 to 66 thereby ensuring the adequate durability at less shot peening cost. As a result, the rollers and discs are less susceptible to fatigue, accomplishing long lives.
Under such conditions as to raise a problem associated with cracks produced in the peripheral surfaces of the rollers, the compressive residual stress may preferably be not less than 0.9 GPa. It is preferred that a hardened layer (portion hardened by the shot peening) may extend in depth of not less than 120 xcexcm from surface.
In the above toroidal continuously variable transmission, it is preferred that the heat-resisting bearing steel contains:
C: 0.8 wt %-1.5 wt %
Si: 0.5 wt %-2.0 wt %
Mn: 0.3 wt %-2.0 wt %
Cr: 1.3 wt %-1.98 wt %
Mo: 0.3 wt %-1.0 wt %,
provided that a total amount of Si and Mo is 1.0 wt % or more, the other components including iron and unavoidable impurities.
In this case, superior durability under high-temperature contact conditions can be achieved because of the use of the heat-resisting bearing steel particularly excellent in high-temperature strength. This combines with the effect of the surface hardening by shot peening to present excellent durability under contact conditions of high temperature and high shear stress which are specific to the toroidal continuously variable transmission.